1. Field of the Invention
The invention relates to a method of and means for reducing the convergence time of an echo canceller connected in a transceiver arrangement between one-way transmit and receive paths coupled to a two-way path and used to cancel an echo signal produced in the receive path in response to a signal supplied to the transmit path. Such an echo canceller comprises a transversal filter having N controllable coefficients for processing a signal derived from the signal supplied to the transmit path and a circuit for producing a difference signal which is the difference between two signals which are formed from the signal in the receive path and the output signal of the transversal filter, respectively.
2. Description of the Related Art
Echo cancellers are used, for example, in data transmission modems whose transmit and receive paths, together forming a four-wire access, are often coupled by a circuit known as a hybrid coupler in manner such that each modem has a two-wire access to an external communication line. It is known that, when establishing a connection between two modems by their two-wire access, an untimely signal may be produced in the receive path of each modem, said signal termed an echo signal, which is a fraction of the signal in the transmit path of the same modem and which is caused by imperfections of the coupling circuit and/or signal reflections in the connection. An echo canceller has for its object to automatically cancel this echo signal in order to permit simultaneous full-duplex transmission between two modems connected by their two-wire access.
In an echo canceller, the signal attenuating coefficients of the transversal filter are controlled to minimize the mean square value of the residual difference signal or error signal appearing at the output of a subtractive difference circuit. When the echo canceller has converged, the coefficients of the transversal filter are substantially equal to samples of the impulse response of the echo path and the transversal filter provides an echo cancelling signal substantially equal to the echo signal at the sampling instants.
A disadvantage of the known echo cancellers is that their convergence time is generally long. The filter coefficients are iteratively corrected by using a gradient algorithm, according to which the correction term of a coefficient at each iteration is the product of a weighting coefficient .alpha. less than 1, the difference signal and a transmitted datum. In the prior art echo cancellers, this iterative mode of coefficient control is used both during the initializing period of the coefficients and during the tracking period in the course of the data transmission proper; see, for example, in this respect the articel of Kurt H. Mueller, entitled: "A new Digital Echo Canceller for Two-Wire Full Duplex Data Transmission", published in IEEE Transactions on communications, Vol. COM-24, No. 9, September 1976, pages 957 to 962. During the follow-up period, in order to avoid that during full-duplex transmission the echo cancelling process will be disturbed by the data signal originating from the remote transmitter and superimposed on the echo signal in the error signal, a weighting coefficient .alpha. of very low value is used, which implies small corrections of the coefficients and a very long convergence time amounting to several seconds, which may adversely affect the tracking possibilities of the echo canceller.
For instance from the aforesaid article, it is known to transmit a particular sequence known as maximum length sequence during the initializing period in order to produce an echo signal and to correct iteratively the coefficients of the transversal filter by using a weighting coefficient .alpha.=1/N, N characterizing the maximum delay produced by the transversal filter, said fixed weighting coefficient leading to the fastest convergence possible. It is also known to improve this iterative correction process in order to reduce to some extent the convergence time during the initializing period by using a weighting coefficient .alpha. which is variable in the course of said period, taking several decreasing values. At the beginning of the initializing period, the echo signal is relatively high with respect to the noise which includes the signal from the remote transmitter and a weighting coefficient .alpha. of high value may be used, which permits a fairly large correction of the coefficients, whereas towards the end of said period, where the echo signal tends towards zero, a small weighting coefficient .alpha. has to be used, which provides only a small correction of the coefficients of the transversal filter.
It will be obvious, however, that even after this improvement an iterative method of coefficient control during the initializing period cannot lead to a very fast convergence of the echo canceller since the coefficients tend asymptotically towards their optimum values and towards the end of the initializing period substantially the same conditions prevail as in the tracking period with coefficient corrections which are necessarily very small.